The present invention is directed to an improvement in axial flow fans of the type having a plurality of rotating impeller blades and used for circulating a relatively large volume of air, for example for blowing air through an air circulation duct.
The abatement of noise in axial flow fans has been a longstanding problem. There are two types of noise produced by the flow of air (as opposed to fan motor noise or other possible mechanical sources of noise) through an axial flow fan: vortex or turbulence noise and rotational noise. Turbulence noise is generally produced as broad band, background noise, and except at unusually high sound levels is not particularly annoying. In contrast, rotational noise, which is produced by the rotating pressure fields of the individual blades of the rotor, tends to produce audible noise at discrete frequencies. Such noise is tonal in character, and can be annoying even when its sound level is not excessively high. The presence of noise which is concentrated at discrete, audible frequencies, i.e. tonal noise, raises the perceived level of fan noise as compared with a fan having the same overall level of noise spread out evenly over the frequency spectrum.
From past studies, it is known that tonal fan noise is produced at frequencies dependent on the number of blades and the speed of rotor rotation, and results from discontinuities, or pressure pulses, produced by the moving blades. Tonal fan noise is generated with particular intensity at the blade pass frequency, a fundamental frequency characteristic of the impeller construction and of blade rpm. Total noise is also produced at harmonics (multiples) of the fundamental blade pass frequency.
An attempt to attenuate tonal or "perceived" noise is described in detail in an article entitled "Controlling The Tonal Characteristics Of The Aerodynamic Noise Generated By Fan Rotors", R. C. Mellin and G. Sovren, ASME Journal of Basic Engineering, 69-WA/FE-23. In the Mellin et al method, the rotor blades are spaced unequally in a pattern selected to reduce the noise level peaks occurring at the fundamental blade pass frequency and at several of the prevailing harmonics, as compared with equally spaced rotor blades. Preferably, the selected blade spacing pattern is such that no two adjacent blades overlap, i.e. that there is at least a minimum gap between even the two most closely spaced blades, in order that the fan may be fabricated using a conventinal axial-draw type of casting. A similar approach, in which the blades are spaced unequally and also the blade angle is varied, is disclosed in U.S. Pat. No. 4,253,800 to Segawa et al.
Another approach for effecting noise abatement in axial flow fans is to use a sound trap positioned at the discharge side of the fan. In one such design, a sintered metal filter is attached concentrically to the fan for absorbing a portion of the fan noise. While such a filter can ideally suppress tonal noise, in use the sintered metal screen is vulnerable to clogging, which may produce irritating, high level discrete frequency tones. Such a filter may also reduce the pumping efficiency of the fan.